Polymers produced by free-radical high pressure technology have wide ranges of applications, for example food packaging, blown and cast film, and extrusion coating. The properties of polymer are designed for the specific applications. For example, polymer materials for film, foam and extrusion coating applications require molecular weight distribution ranging from narrow to very broad, melt strength and/or melt elasticity ranging from low to high, while in film applications good optics have to be maintained. Furthermore polymers (LDPE and high pressure copolymers) produced by the free radical high pressure technology are increasingly used to improve processability of linear low density polyethylene polymers (LLDPE) made by coordination catalysts. Typically LLDPE lacks melt strength due to the low level or absence of long chain branching and is difficult to extrude. By blending in LDPE, processability and line speed in the application can be significantly improved and fabrication can occur at improved economics.
It is well-known that conventional low density polyethylene (LDPE) is produced with high pressure (for example, 160 MPa to 400 MPa) technology, either in autoclave and/or tubular reactors. The initiator systems, known as free-radical agents, are typically injected at multiple points along the reactor, thus creating multiple reaction zone system. The polymerization usually takes place by feeding free-radical initiator system(s) at temperatures in the range from 130° C. to 360° C. In addition, a chain transfer agent (CTA) is used to control the molecular characteristics of the polymer product. It is known that feeding preferentially “make up CTA” to an ethylene inlet (front) feed stream or to a side ethylene feed stream will respectively narrow or broaden the molecular weight distribution (MWD) of the polymer products, see U.S. Pat. No. 3,654,253. Due to the relatively low conversion level/or activity of commonly used CTAs, a significant amount of the CTA fed to the reactor, is recycled back to the polymerization reactor system, wherein CTA is pro rata distributed over all the ethylene feeds towards the reactor. The “make-up CTA” is added to the reactor feed streams to maintain the correct level of CTA in the polymerization, needed to control the product melt-index. The amount of “make up CTA flow,” which depends on the CTA conversion level in the reactor and other losses, such as purge, residual CTA in product, and/or condensation in the recycle and compressor sections, typically varies from 1 to 20 percent of the total amount of CTA added to the polymerization. The location of the feed of the “make-up CTA” can be used to vary the concentration ratio of “the concentration of CTA in the front ethylene feed stream” versus “the CTA in the side ethylene feed stream[s].” Moreover, “make-up ethylene” is added to a reactor to compensate converted and lost ethylene (via the purge, etc.). Typically, make-up ethylene is added via the feed streams to the suction (inlet) of the hyper (secondary) compressor system after passing to a booster and/or a primary compressor system. The conventional methods of feeding CTA using the primary and/or booster compressor result in a limited variation of CTA concentration in the reactor feed streams, and these variations are especially limited with CTAs of low activity (for example, see US Publication No. 2003/0114607).
DD 276 598 A3 (English translation) discloses a process for adjusting and regulating the input gas streams for multi-zone tubular reactors, with at least two side input streams, for the production of ethylene polymers, by free-radical bulk polymerization, and in the presence of 10 to 50 ppm of oxygen, as polymerization initiator. Also disclosed is a two-step venting of the reaction mixture in an intermediate-pressure product separator, and in a low-pressure product separator, and a separation of the polymer, and returning the unreacted reaction gas to the cycle. A chain regulator and make-up ethylene are added to the low pressure return gas. The resulting gas stream is divided into two gas streams, in a ratio of 2:1 to 1:4, and to one of the gas streams is added oxygen, in an amount from 50 to 500 ppm, and the two gas streams are separately compressed to the intermediate pressure. The distribution of oxygen and CTA are linked to each other by which the CTA distribution over the reaction zones cannot be independently controlled and separated from the maximum temperature control in the reaction zones.
US Publication No. 2010/060244 describes a process to make LDPE products by injecting make-up CTA into multiple positions of the reactor in which the ratio of Chain Transfer Agent in the first reaction zone over the ith reaction zone (Z1/Zi) is always less than 1. US Publication No. 2011/052525 describes a process to make LDPE products by injecting make-up CTA into multiple positions along downstream of the reactor in which the ratio of Chain Transfer Agent in the first reaction zone over the ith reaction zone (Z1/Zi) is always greater than 1. US Publication No. 2012/059469 reports different high pressure polyethylene reactor configurations where ethylene (make-up and a recycle flow from low pressure separator) is lined up to different reaction zones. The publication discloses that different line-ups for various reactor configurations resulting in discrete Z1/Zi—values.
WO 2012/117039 Publication discloses a process for preparing polyethylene in the presence of free-radical initiator and Chain Transfer Agent (CTA), wherein major part of make-up ethylene is fed to the front of the reactor to maintain the concentration of CTA in the first reaction zone at less than 70% of the highest CTA concentration of other reaction zones. It also discloses zero CTA concentration in the first reaction zone wherein the major part of fully make-up CTA as well as high and low pressure separator flows are recycled to the following reaction zones (or the side of the reactor).
The above cited art to influence MWD by varying the CTA concentration along the reactor results in either limited variation, when this is achieved by distributing the make-up CTA, or in discrete distributions or distributions in discrete narrow ranges when different line-ups of make-up ethylene are used.
The state-of-the-art conventional polymerization processes are very limited in terms of preparing polymer products with a broad range of molecular distributions and a broad range of melt strengths at a given melt index. It is noted that narrow MWD products are typically made at reduced polymerization temperatures and therefore reduced conversion levels which clearly indicates more expensive products. Therefore, it is important to develop new polymerization processes, by which the CTA concentrations in the reactor can be varied widely, and most preferably independent from the activity or conversion level of selected CTA system and which are able to generate ethylene-based polymers at high conversion levels with a wide range of molecular weight distributions (MWDs) and/or a wide range of the melt elasticity and G′, at a given melt index. There is a need to produce at the same reactor temperature and pressure conditions broad as well narrow MWD polymers at high conversion levels. Furthermore there is need to control the MWD and melt elasticity of specific polymer resins without modifying reactor conditions like polymerization temperatures and inlet pressure and creating secondary effects on product properties like short chain branching and unsaturation formation. There also a need to develop a control system for controlling the impact of the make-up ethylene distribution and/or distribution of the make-up CTA on polymer properties. These needs are described in below invention.